1. Field of the Invention
The present invention relates to an automotive AC dynamo-electric machine capable of bidirectionally transmitting motive power to and from an engine via a power transmission device, and also capable of starting the engine by implementing a motor operation mode and of supplying electric power to a vehicle by implementing a generator operation mode.
2. Description of the Related Art
FIG. 14 is a sectional view showing a conventional automotive AC dynamo-electric machine (hereinafter referred to as a "dynamo-electric machine"). The dynamo-electric machine includes a bracket 80, a Lundell-type rotor 200 secured to a rotor shaft 12 provided in the bracket 80, a stator 101 secured to an inner wall surface of the bracket 80, a slip ring 15 secured to an end of the rotor shaft 12 and which supplies current to the rotor 200, a pair of brushes 13 that slide on the surface of the slip ring 15, and a brush holder 102 that accommodates the brushes 13 and springs 14 pressing the brushes 13.
The rotor 200 has a field winding 11 that generates magnetic fluxes as current flows therethrough, and a rotor iron core 10 that covers the field winding 11. The stator 101 has a stator iron core 1 through which the magnetic fluxes from the field winding 11 pass and which is composed of a plurality of layers of steel plates, and a three-phase stator winding 2 through which three-phase alternating current passes.
An operation of the dynamo-electric machine having the aforesaid construction will now be described.
When the engine is started, the dynamo-electric machine acts as a motor to supply AC current to the stator winding 2. This generates torque in the rotor 200, and the torque is transmitted to a V-belt (not shown) wound on a pulley 60, which is a component of the power transmission device, thus starting the engine.
When electric power is supplied to a vehicle, the dynamo-electric machine acts as a generator. The power from an engine rotates the rotor 200 via the pulley 60 and the rotor shaft 12. At this time, current is being supplied from a battery (not shown) to the field winding 11 of the rotor 200 via the brushes 13 and the slip ring 15, and therefore magnetic fluxes are being generated. The rotation of the rotor 200 causes the magnetic fluxes to interlink the three-phase stator winding 2, and an electromotive force is generated in the three-phase stator winding 2, supplying electric power to the vehicle.
In the dynamo-electric machine having the construction set forth above, a pulley ratio (a ratio of an engine pulley diameter to a dynamo-electric machine pulley diameter) in the belt of the power transmission device is set to about 2 at maximum in order to prevent slippage of the belt and to secure a sufficient dimension of the pulley 60 for winding the belt around it so as to securely transmit the torque to the engine when starting the engine.
In the above dynamo-electric machine, a ratio of a number of revolutions of the dynamo-electric machine to a number of revolutions of the engine in both operation modes remains unchanged. When the pulley ratio is considered, in order to obtain adequate rotational output required for starting the engine, the dynamo-electric machine must be made larger because of an expression shown below, presenting a problem in that the dynamo-electric machine becomes too large to be mounted in a vehicle:
Rotational output (W) .varies.(Outside dia. of rotor).sup.2.times.Volume of stator iron core=Volume of dynamo-electric machine